The first version consists of bottle caps glued to the inside back of the underpants, covering the area of both butt cheeks that would be in contact with a chair when sitting. The bottle caps are glued close together with the top (smooth side) glued to the underpants, but not so close that they snag on each other. Since there is not enough elasticity in the underpants to hold the bottle caps firmly against the butt cheeks, I wear an old pair of my wife’s control panties (half way between normal panties and a heavy girdle. In addition to the constant reminder that you have them on, when you take them off you also get a nice bottle cap pattern on your butt cheeks that lasts for several hours.
The second consists of thumb tacks covering the same area as the bottle caps. The thumb tacks are pushed through from outside to inside so that the points face the butt cheeks.
On the inside, I line the area to be covered with a thin piece of soft foam rubber (about 1/8 inch thick) that I salvaged from some computer packing material. That is used to help keep the tacks straight and also to soften the effect when desired. While inserting the tacks, I use a piece of stiff foam rubber inside the underpants so that the tacks stay put while I work. After all of the tacks have been inserted, I cover the outside back of the underpants with duct tape to keep the tacks in place. If you own a TENS or other such device, you can place a piece of aluminum foil, folded several times, over a portion of each butt cheek, running it vertically to above the area covered by the duct tape. This will give you two electrodes to connect to and some interesting results. 
Note, however, that you cannot cover a large percentage of each butt cheek with the foil or the duct tape will not hold the tacks on the underpants properly. If you want a stronger reminder of your session, you can remove the inner foam. Either way, unless you impact the tacks hard (falling on your butt, flogging, etc.), you should not get punctured as long as you place the tacks close together. It is much like the bed of nails trick. I also wear an old pair of my wife’s control panties with these for the same reason as above.
